Seismic Data Inversion
The accumulation of petroleum in the subsurface is influenced by many subsurface geologic factors, including rock type, lithology, age, depth of burial, porosity, permeability, fluid content, and structure. Information about any of these can be of great value to oil finders and producers.
The most reliable source for this information is by direct examination and testing of rock samples obtained from well cores or cuttings. When rock samples are not available, borehole logs may be used to obtain subsurface information by indirect means. Some of the physical properties of rocks can be measured and then interpreted in terms of geologic information. Such log measurements can also be used to map structure and facies changes by correlation of formation tops across a suite of logs from several boreholes.
Mr R.E. Wyman, in his excellent AAPG Continuing Education Lecture, demonstrates the links between geology, geophysics, and several petrophysical measurements, by use of a “Rosetta Stone” which compares several types of measurements of the same geologic column.
One type of log, the sonic, yields a continuous profile of the transmission velocity of sound through the rocks which make up the geologic column. It is one of the more useful log types because, as has been shown by Faust ( 1951 ), rock velocity varies directly as a function of three important geologic components; lithology (including porosity and fluid content), age, and depth.
Unfortunately, an expensive borehole must be drilled to obtain either rock samples or log information. The only presently known form of